This invention relates to an injector used for feeding feedstock material into the axis of a jet of heated gas.
Thermal spraying is a coating method wherein powder or other feedstock material is fed into a stream of heated gas produced by a plasmatron or by the combustion of fuel gasses. The feedstock is entrapped by the hot gas stream from which it is transferred heat and momentum and it is impacted onto a surface where it adheres and solidifies, forming a relatively thick thermally sprayed coating by the cladding of subsequent thin layers or lamellae.
In the case of some thermal spray applications, injecting feedstock axially into a heated gas stream presents certain advantages over traditional methods wherein feedstock is fed into the stream in a direction generally described as radial injection, in other words in a direction towards the axis of the gas stream. The advantages of the axial injection relate mainly to the potentitto control better the linearity and the direction of feedstock particle trajectory and to increase its velocity. However, this has been accomplished in the past by interposing a core element through which feedstock is injected axially. Although the fundamental principle of wrapping a gas flow around a core member appears to be a desirable way of achieving axial injection, in practice the core causes significant turbulence of the gas stream. It would be therefore desirable to inject feedstock in a manner that achieves an optimal particle trajectory in the axial direction by inducing minimal turbulence of the gas stream.
Plasma torches with axial injection of feedstock can be classified in two major groups: a) those with multiple cathodes, also known as the pluri-plasmatron or the multiple-jet type and b) those with single cathode, also known as the single jet or single electrode type.
Examples of multiple cathode plasma torches with axial injection are found in U.S. Pat. Nos. 3,140,380 of Jensen, 3,312,566 of Winzeler et al., 5,008,511 of Ross and 5,556,558 of Ross et al. They show a plurality of plasmatrons symmetrically arranged about the axis of the plasma spray torch and provide for nozzle means to converge the plurality of plasmas into a single plasma stream. Feeding means are also provided to inject feedstock materials along the axis of the single plasma stream. This type of plasma torches involve complex torch configurations with increased chances of malfunctioning and require the use of multiple power supplies for powering the multiple cathodes. The use of multiple cathodes and multiple arc chambers, which need to be replaced regularly, induce high operating costs for such plasma torches. A different approach to achieve axial injection employing multiple cathodes and a complex single arc chamber configuration is found in U.S. Pat. Nos. 5,225,652, 5,406,046 and 5,332,885, all three issued to Landes.
The single cathode type plasma torches with axial injection have certain advantages over multiple cathodes systems such as less complex torch configuration and reduced operating and manufacturing costs. Typical arrangements for the single cathode approach are found in U.S. Pat. Nos. 4,540,121 of Browning, 4,780,591 of Bemecki et al., 5,420.391 of Delcea, 6,202,939 of Delcea and 5,837,959 of Muehlberger et al.
U.S. Pat. No. 4,780,591 of Bemecki et al. teaches the semi-splitting of the plasma stream by means of a core member positioned axially within the feedstock injector and a plasma splitting arm which extends from the core to the injector internal wall, defining a xe2x80x9cCxe2x80x9d shaped plasma channel. The feedstock is injected axially through the core member. As shown in FIG. 1 of the drawings, this approach creates an asymmetrical plasma stream flow within the injector, with a portion of the plasma stream going around the core member, while the arm splits the other portion of the stream. Apart for the obvious asymmetry, this particular type of flow dynamics creates a flow conflict that induces asymmetrical jet turbulence.
U.S Pat. No. 5,420.391 of Delcea also teaches a core member positioned axially but instead of providing only one arm as in Bernecki ""591, two or more splitting arms now extend from the core member to the outer walls, defining kidney-shaped plasma channels arranged symmetrically around the core, as shown in FIG. 2.1. This arrangement allows the symmetrical wrapping of the gas flow around the core member. Similarly, U.S. Pat. No. 5,556,558 of Ross teaches kidney shaped plasma channels arranged in an encircling relationship around a core member but instead of splitting a single plasma stream, Ross provides for independent plasma jets for each of the plasma channels. Inherent to the design in Delcea ""391 and in particular when only two plasma channels are provided, each channel has plasma-shaping walls defining essentially a kidney-shaped cross-section in order to accommodate either a cylindrical or a conical core member between the channels. A plasma torch having a single gas stream with circular cross-section flowing around a central core member suffers two fluid mechanic transformations while passing through the internal pathways of the injector, i.e. firstly the splitting of the stream into a plurality of streams around the core and secondly the volumetric transformation as each of the split streams conforms to the shape of the kidney shaped channels encircling the core member. When leaving the injector, the split streams must be merged smoothly into a single stream having again an essentially circular cross-section. The region where the split streams merge (which is also the region where the feedstock is injected into the stream) becomes quite turbulent, causing non-axial feedstock trajectories within the merged stream. According to fluid mechanics theory, turbulence is generated inside each of the splitting channels due to gas flow separation occurring along the walls of the core and of the channel cavities adjacent to each splitting arm. This gas flow separation is caused by adverse pressure gradients due to the forced shaping of the split stream around the core member. The flow turbulence at region of feedstock injection introduces non-axial velocity vectors causing random feedstock trajectories, resulting in molten feedstock adhering to, and solidifying on the internal wall of the output nozzle with the consequent malfunctioning of the spraying process. These phenomena are shown schematically in FIG. 2.1 and FIG. 2.2 of the drawings. FIG. 2.1 for example, shows the two opposed cross-sectional flow gradients induced within each plasma channel due to the kidney shaped flow around and about the central core member. The effects are as follows: a) plasma gas turbulence due to the opposing directions of the flow and the counter-flow gradients induced within each converging channel (only one type of flow gradient is shown in each channel in FIG. 2.1) and b) plasma gas turbulence due to the gas flow separating (detaching) from the splitting arms and core surfaces. Consequently, the feedstock is injected into a non-laminar and turbulent flow, resulting in at least some percentage of the feedstock particles attaining non-axial trajectories. This directs a portion of the feedstock particles towards the inner wall of the output nozzle, resulting in the build up of molten deposits on the inner wall of the output nozzle and possibly on the feedstock injection tip itself. The nozzle build-up phenomenon is shown schematically in FIG. 2.2.
This xe2x80x9ckidney shape effectxe2x80x9d can be reduced to some degree in Delcea ""391 by providing an increased plurality of plasma channels as shown schematically in FIG. 2.3 of the drawings. For example, if six or more channels were provided, their cross-sections would shrink to become more or less circular or slightly oval. This approach would result in a proportionate increase in the number of splitting arms as well as an increase in the total surface area of the internal pathways exposed to the hot gas. Consequently, the conduction heat losses would also increase accordingly, therefore rendering the injector thermally inefficient.
One way of partially addressing the problems in the torch of Delcea ""391 while using only two plasma channels is as shown in U.S. Pat. No. 6,202,939 of Delcea. Delcea ""939 also provides a core member and two connecting arms, with the core being encircled by two kidney shaped channels. Two small holes are provided in the core diverting a small portion of the gas stream into the feedstock input channel to increase the axial injection effect and therefore to overcome some of the flow turbulence generated at the region of feedstock injection.
In the case of thermal spray torches, it is common practice to attach a flow expansion output nozzle in order to increase feedstock velocity and the transfer of heat to the feedstock. As a general rule, the longer the output nozzle the more heat and velocity is transferred from the gas stream to the feedstock and therefore denser thermal spray coatings can be obtained. One of the main factors that limit the length of the output nozzle is the trajectory of the molten feedstock along the nozzle passage. If the injection of the feedstock is such that at least some of the feedstock will deviate towards the internal wall of the nozzle solidifying and building up on the cold surface of the wall it will result in the malfunctioning of the spray torch.
One of the most significant problems affecting the prior art single stream plasma torches with axial injection is xe2x80x9cspittingxe2x80x9d due to the turbulent contacting of the feedstock by the gas streams. xe2x80x9cSpittingxe2x80x9d is a periodic burst of released feedstock from the outlet end of the torch when some feedstock which has solidified on the internal pathways of the torch such as on the output nozzle inner wall or on the feedstock injection tip is subsequently remelted by the heated gas and periodically released as relatively large droplets, which become incorporated within the sprayed coating as structural defects.
It would be desirable to provide a superior feedstock injector for attachement to a single stream thermal spray torch, the injector providing for a simplified as well as optimized mechanism for splitting and shaping the single stream with reduced turbulence resulted from the interaction between the stream and the internal pathways of the injector. There is a need for a superior feedstock injector having its internal pathways shaped so as to provide a single step, streamlined splitting mechanism wherein a single gas stream is split in the least intrusive and least turbulent manner, to minimize gas turbulence at the feedstock injection region and to provide an uniform contact of the feedstock with the gas stream.
The present invention provides an axial feedstock injector having an innovative internal configuration that provides a substantially improved gas flow through the injector.